This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A117204 #24 Mar 02 2021 06:00:29 %S A117204 1,2,3,5,6,7,10,11,14,15,17,19,21,23,26,29,30,33,34,35,38,39,41,42,43, %T A117204 46,47,51,53,55,57,59,61,65,66,69,70,71,74,77,78,79,82,83,86,89,91,93, %U A117204 95,97,101,102,105,106,107,109,110,111,113,114,115,118,119 %N A117204 Squarefree positive integers k such that 2*k+1 is also squarefree. %C A117204 The asymptotic density of this sequence is (3/2)*A065474 = 0.4839511484... (Erdős and Ivić, 1987). - _Amiram Eldar_, Mar 02 2021 %H A117204 Ivan Neretin, <a href="/A117204/b117204.txt">Table of n, a(n) for n = 1..10000</a> %H A117204 Paul Erdős and Aleksandar Ivić, <a href="https://users.renyi.hu/~p_erdos/1987-32b.pdf">The distribution of values of a certain class of arithmetic functions at consecutive integers</a>, Colloq. Math. Soc. János Bolyai, Vol. 51 (1987), pp. 45-91. %F A117204 a(n) = (A117203(n) - 1)/2. %e A117204 10 and 2*10 +1 = 21 are both squarefree, so 10 is in the sequence. %t A117204 sfQ[n_]:=SquareFreeQ[n]&&SquareFreeQ[2n+1]; Select [Range[200],sfQ] (* _Harvey P. Dale_, Mar 12 2011 *) %Y A117204 Cf. A005117, A117203, A117206. %Y A117204 Cf. A005384, A111153, A065474. %K A117204 nonn %O A117204 1,2 %A A117204 _Leroy Quet_, Mar 02 2006 %E A117204 More terms from _Jonathan Vos Post_, Mar 03 2006 %E A117204 Corrected and extended by _Harvey P. Dale_, Mar 12 2011